Adjusted my valve clearance recently prior to solving the air flow meter problem.

All my inlets were tight (between .2 and .25 mm) and exhausts were fine. So I shimmed the inlets to spec (.3mm cold). Surprise surprise there was no noticeable difference. Backfire remained when changing from second to third.

If however your valve clearances have increased (running rich with carbon build up?) then this may effect the breathing of the engine. Still suspect your problem is fuelling though.

Worth checking clearances - took me about 2 hours.

Cheers

Ian

Hmm - hadn't thought of that. Backfire is not always present, most times it's very subtle. Valves last adjusted 20 mos/11k miles ago. PO had a dealer do it, so who knows what kind of a job they did. Will look into it... Thanks.

My M6 did exactly as you describe, except did not ever backfire. When cold, slow throttle response until fully warmed up. I suspected all the things you've mentioned to no avail. Fixed the problem by having the valves set by a very talented mechanic that owns an 88M5. Engine runs like a dream now. HTH

Well, car acted up again with the swapped AFM, so that's not the problem. Here's a real clear description of what it did: I'm in 1st, apply throttle, car acts like it is pulling a 2 ton weight, RPMS go up but reluctantly and car just doesn't want to go. I keep it 4000 rpms, car goes but again, I can feel I have a lot more throttle applied than I normally would to drive at a constant 4000 in 1st. Suddenly, after 20 secs or so, rpms zoom up past 5000 and car of course accelerates (without me moving the throttle - in fact I almost have to back off on throttle lest it redline). I shift to 2nd, and boom there is a small backfire when throttle released during shift. Then it drives fine. This ONLY happens when engine is stone cold and I drive away immediately. It also seems to only happen when the air temperature is say 45-70 degrees??!!? SO seems temp dependant, but I've put in a new coolant temp sensor, and swapped an AFM, which of course has the air temp sensor.
I have also checked the coolant temp sensor at the ECU plug, seems fine.

This is driving me nuts, my mechanic is baffled. Still offering a beer shipment (your choice) to anyone who hits upon a possible cause that I haven't tried...